Harmonic generating system



Oct. 1, 1946.. J, W- MCRAE v2,408,437

u HARMoNIGvGENERATING SYSTEM Filled Oct. ll, 1941 Z'SheetS-Sheetl NID ATTORNE V l /NVENTOR ocr. 1, 1946. J. w. MCRAE 2,408,437v

A HARMONIC GENERATING SYSTEM Filed oct. 11, 1941 2 Sheets-Sheet 2 62 fl/VVENTOR HVJ. WMe RAE ATTORNE V Patented Oct. 1, 1,946

.UNITED .STATES PATENT orties' f y Y Y Y 2,408,437 v I I VAmimiioN-IoGENERATING s-YsTEM James W. McRae, Neptune, N, J., 'assigner to'BellTelephone Laboratories, 'Inrpo'ratei New. York, N. Y., a corporation ofNewYork Application october i1, 1941,1se`ria1Nof4-14t95 v8 Claims.

This invention relates `to 'harmonic generating or lfrequencymultiplying systems and more particularly to those employing streams ofcharged particles, e; g., electrons, together with hollow resonators orWaveV guides especially at ultrahigh frequencies. l v Y An object of theinvention is to provide i-ncreased outputs of ultra-high frequencypower, at frequencies above thepractical operating limits of oscillatorsand amplifiers already available in the art.

A related obj ect is the efficient .transfer of power from a beam ofelectrically chargedparticles, e. g., electrons, to an ultra-highfrequency wave or current in a transmission Eline, wave guide or thelike.

Another object is to effectively excite electromagnetic .oscillationswithin a reson'ating chamber of very small dimensions, such as maybedesigned to resonate Iata wave-length of a few centimeters or less, bymeans of an electron beam or cathode ray deflectedy or rotatedperiodically ata frequency .relatively low compared with the reso@ nantfrequency vof the chamber.

.In addition to other uses the invention may be employed to multiply thefrequency of an electromagnetic wave--after it has been subjected tofrequency modulation or frequency vstabilization or other process at arelatively low frequency where'the necessary techniques of the latteroper- (ci. srs-Sl ations are more readily available than at the desiredfinal frequency.

The invention is more fully described 'hereinafter with reference totheacconpanying drawings illustrating a number of embodiments, while thescope of the invention is defined 'in the appended claims.

Inv the drawings: Y l y Fig. l shows an arrangement in which anVelectron beam swinging 'in a'plane passes through a resonating chamberduring a small portionl of each cycle of the oscillation;

Fig. 2 shows an arrangement in which an elec'- tron beam is swung aroundcontinuously to describe a conicalsurface and a reaction Vb'c'etv'veenthe beam and 'an associated resonator occurs -several times during' eachrevolution of the beam;

Fig, ZA'is an elevational View of 'the resonator in the system of Fig.2;

`Figf is a diagram'useful in developing design formulae for the systemdisclosed in Fig. A2;

Fig. 4 is anelevational and'somewhat diagrammatical view of 4a resonatorin the form of a `wave b'od-ying the invention;v

Fig. 5 is an enlarged cross sectional fragmental view f a wave guidesuch as that shown in Fig. 4; `and Fig. 6 shows .an embodiment that isin some respects a VIllodii'cat'ion of ther 'system illustrated Y inFig.. 2`.

'Referring to Fig. 1, there is shown a vacuum tube with an .insulatingair-tight envelope I U ion-'taining' an"electron gun, indicatedgenerally at II I, andra'rilelectron intercepting electrodeor colletorI2. Ab'lock I3 of conductive material Such as copper" is shown fusedinto the envelope i0.' The block I3 is hollowed out to forni .aninternal resonating chamber I4 with smooth, highly conductive innerwalls, the space Within the' chamber communicating with the interior ofthe envelope I through an entrance aperture I5` andan exit 'aperture I6.The axis of the electron gun IjI 'of the collector I2 and of theapertures f5 :and 16 are arranged colinearly so that an electronbeamemitte'd by the gun may be passed through the chamber I4 by way ofthe apertures I5 land I6 to the collector I2. A pair of deecting platesII and I8, supported in any suitable manneras by rods lextending throughthe yenvelope I 0, are mounted on either side of the common axis andiconnected respectively to the two terininalsfof a source I9 rof, highfrequency waves. A pair' ofshielding plates 20 and 2| with their 'edgesseparated to form a slot 2'2 are mounted on either side of the axis andboth are conductively coupled to a relatively low voltage source 23 ofsubstanti'ally constant positive biasing potential. A pair ofjbeamfoc'using plates 24 and 25 Vare mounted one Von' either side oftheaxis at a position along thev course of the beam kbeyond the shieldingplates 2li and 2|. vThe plates 24 and '25 are conductivelyv coupledtovthe negative `terminal of a source 23. The block I3isconductivelyc'onnected 'with a hollow conductive `pipe or wave guideQ26, the interior of which `corn'municates with the chamber I4` througha passageway 21 hollow'ed out of the block. A suitable air-tight orvacuuniseal Vis provided by a small bulb 28 or bead of 'in'sulating'Amaterial fused to the block I3. A source 29 o'f'relatively high positivebiasing potentialis connected 'between the plates 2li, v2| andtheconductive mass comprising the block I3 and Athe wave guide 26, thepositive terminal of the source .2'9 being connected to the .lattersystem, and grounded vif desired. The collector I2 may also be connectedto the system comprising the block I3-and guide26f. The heating elementwithin the electron gun I I" may be energized in any vsui-table manner,l'as for example, by a source 30 of electromotive force connected byleads 3l and 32 to the appropriate terminals of the gun. By-passcondensers 33 and 34 may be shunted across the source I9 and the commonterminal of the condensers may be connected to the lead 32 to fix theaverage potential of the plates I'I and I8. An electron beam controllingelectrode Within the gun I I may be connected by means of a lead 35 tothe positive terminal ofthe source 23 to determine the current strengthof the electron beam.

In the operation of the system of Fig. 1, the electron beam is swung upand down in a vertical plane by the action of a high frequency waveimpressed upon the plates II. and I8 from the source I9. Twice duringeach cycle of the oscillations the beam lies in the axis anda pulse orgroup of electrons is projected through the resonating chamber i4 by wayof the aperturesV I 5 and I6. The electron pulses or groups, if properlytimed, serve to sustain electromagnetic oscillations within the chamberI4, Evidently the timing will be correct if the pulses are made toarrive at intervals of an integral number of cycles of the oscillationsin the chamber I4. For example, the frequency of the source I9 may be500 megacycles per second and the beam may be made to sustainoscillations in ka suitably adjusted resonating chamber at a frequencyof 10,000 megacycles per second, in which case one electron pulsetraverses the chamber for every 10 complete oscillations of the iieldWithin the chamber.

The physical dimensions of a resonating chamber or cavity designed tooscillate in the fundamental mode at 10,000 megacycles per second ormore, corresponding to 3 centimeters or less in wave-length, arenecessarily very small. Furthermore, the length of the gap, designatedby a as. shown in Fig. l, traversed by the electrons, must be veryminute. This is because the electron transit time in the gap should beshort and preferably not more than a half cycle at the resonantfrequency of the chamber, or'in other words a transit angle of not over180 degrees. If it is desired, for example, that the electrons traversethe gap in a transit angle of only 75 degrees, and if the speed of theelectrons corresponds to 2400 electron volts, then for a 3 centimeterWavelength resonator, the dimension a of the gap should not exceed about0.025 inch. The hole through which the beam passes, the diameter ofwhich is designated b in Fig, l, should also be small in proportion tothe principal dimensions of the cavity i4. In the case of a 3 centimeterWave-length resonator b should not much exceed 0.05 inch. If then, abeam of 0.05 inch diameter is focused to pass through the cavity withzero deflection voltage on the plates I I and I8, the application of asinusoidal deflecting voltage to the plates will result in a short pulseof current entering the cavity at every instant of zerodeflectingvoltage. The duration of this pulse should evidently be lessthan half a cycle of the resonant frequency of the chamber` in orderthat the eld -in the resonator may not return energi7 to the electronsduring part of the cycle. In the example, under consideration, the timeof a half cycle is` 1/20,000 microsecond. In order for the beam to crossthe aperture within the period of a half cycle, the linear velocity, v,of the beam in the-ver tical direction must be given by iehes persecond. .This'vehie ef @is .equal i0.

- across the aperture I5 and will be referred to hereinafter as thewriting velocity of the beam, from the tracing or writing motionexecuted by the beam.

The required value of the velocity o together @with the frequency of thedeflecting source I9 serves to determine the maximum amplitude ofdeflection which must be imparted to the beam.

y that of the longitudinal velocity.

For the purpose of making this calculation, the deflection of the beamin the vertical direction at the position of the aperture I5 may berepresented by y-:A sin et (2) where A is the maximumamplitude to becalculated and w is 2r times the deecting frequency.

The instantaneous velocity of the beam is determined by %=Aw COS cui (3)from which it appears that the maximum writing velocity is Aw. In thenumerical example under consideration It is also simple to calculate theapproximate voltage required upon the plates Il and I8 to effeet anamplitude of deflection of 0.64 inch. If, for example, the distancealong the axis of the tube from the deecting plates II, I8 to theaperture I5 is taken as 6A, or 3.82 inches, the lateral velocity whichmust be imparted to the electrons at maximum deiiection is one-sixth Inthe assumed case of a 2400 volt beam, the deflection may be effected by1/36 of 2400 volts or approximately 67 volts. If desired, the amount ofdeiiecting voltage required may be reduced by increasing the length ofthe tube, and conversely, a shorter tube will require va greaterdeflecting voltage.

If the high energy beam were allowed to strike the block I3 during allthe time except when it A =0.64 inch A was passing through the apertureI 5, only a small fraction of the total energy of the beam would beimparted to the cavity. An improvement in the efficiency of the deviceis secured by the use of the shielding plates 20, 2| and the focusingplates 24 and 25. The plates 20 and 2I being at a relatively lowpotential with respect to the cathode and serving to shield the .beamfrom the high potential of the source 29 impressed upon the block I3,the electron beam is in effect a low -voltage beam except during a smallinterval when the beam is passing through the slot 22 .between theplates 20 and 2l. Thus during most of the time that the beam is notpassing through the aperture I5, it is composed of low voltageelectrons'which strike one or the other of the shielding plates 20 and2I at low velocity and with correspondingly 10W dissipation of energy.During the small fraction of the time when the beam passes throughtheslot 22, the electrons are accelerated longitudinally bythehighnvoltage upon arca-esc thesblock..1l3. 'Theplates 24 and '25.servetto iocs'fthe' beam .during theiinterval vvh'enV itis passing throughthe .slot 22. .The spent. electrons which emerge romf'the :aperture .I6are collected bythe collector .122. .The vultra-higll frequency wave'Ymaintained .withinirthe chamber. I4 gives .rise to atraveling Wave inthe Wave guide 26-fbyway oi'thercoupling .'aftordedby the passagefZljandmay? be led away itoany desired pointifor utilizati'ori.

. Iln the arrangem'ent'of JEi'g. 112,' .twov .pairs 'of'..de

ectinfglplates iat 'right .angles to each other .are-

providedlatl, |18 and IT., .l8.lre'spectively.. The sou-ree 119 isconnected t0 the vdeilecting' .plates through .a phase shifting .network4D :of any known-suitable design .whiclrprovid'es'two .substantiallyequal voltages 2in :time l:Quadrat-,ure Plates 1.1 and 11". :areconnected' together and also connected to the .cathode'andlto :thecenter `terniiial lof the .r'i'etvvorklLy vThe plates ,1.8 land. I8'are. connected respectively/to .the remainingk terminals lof the network40. The block 'lf3 vfisr'eplaced lby. a somewhat Vsimilar conductivev'block IEW-.having a .hollow cavity'l vof .annularfiorm with a acrosssectional .sha-pe .substantially the sameas that v.of thecavity i4 :inFigfl..V Thezcavity ."Misa gure of -revolutionabout the central axis,'which lies outside the cavity. A .series .of equally spaced entranceapertures 1H `andcorrespending exit aperturesZ vare provided, thearrangementof the apertures .4l being shown more clearly 'inFg 2A. Acollector electrode l2 is provided beyondthe-exit apertures 42.

In the v'operation fof1=the arrangement of Ijig. 2, the electron beamkisgiven a 'rotating .motion by meansof the crossed electriceldsmaintained between 'the pairs of deflecting plates. The elec-v tron.beam generates a .conical surface, sweeping out La circular-trace'onthe surface vof the block i3. The radius .of't'hise'ircleis`'adjusted so that the trace passes approximately through the .centersof the .entrancerapertures 4l. Inthe course of .rotation the beam sendssuccessive 'electron pulses 'through the chamber I4 4by Way 'of theapertures 4l in rotation. Provided theiresonant frequency `of thechamber 2M is equal to :an finteger times the frequency of the source ittimes Y the number of. apertures, a high frequency .electromagnetic wavemaybe maintained inside the cavity 'IN vand ultra-high`frec'luency:power 'deliv ered to the. associated wave vguide 26. Eachexit aperture 42 is aligned with an Ventrance aperture 4.1 and 1anvelement .of the .conical .surface `generated bythe beam. The minimumdiameter of which the apertures M `lie, may be `determined bycalculationLinra given case. Referring'to Eig. 3, let the 'diameter .ofthe electron beam digand the `diameter of eachV of the holes .in thev'cavity be da Then, if the ratio fof the input frequency to the `outputfrequency is `to be n, andfa pulse is to be delivered to the resonatorforeach cycle of thetharmonic Wave, there vmust be 11. holes equallyspaced .around the circumference. of .a eirclerof diameterD, where v 1duced -out-put for fthe same holediameter be# cause-"of fthe'l-argerfraction or electrons striking the outer .-surface -oftlie resonatorwithout yenthe circle upon tering-it. :.Itwould bed'esirable tohavesthe'b'ea'm diameter much .less than thatrof thefholes, but the :useof .a ism'aller beaml kdiameter .requires a higher'fbeamcurrentdensityiin korder to deliver 'the same amount Yoffp'owe'r .by wayvof .the beam.

Vclot-"dash radial lines.

"iTheembodiment's-of the invention .hereinabove describedfmay generally:be so designed vas 'riot-'to require .the `use 'of 'maximum Writingvelocities equalto orl greater than the :velocity of flight. However,`lasthe writing velocity of 'thel beam "is not" the yvelocity :oranyimaterial body and is not mnerently :limited @to values less than thevelocity o' ght,` illustrative arrangements are describedhereinaiter-'which require -Writih'gV-eloci'ties greater than .thevelocity. of light. 1

One such arrangement .will "be described -by us of. .a 'somewhatdiagrammatic representation in Fig.. `4. t'llhe ligurerepresentsfalength oihol'low, conductvawalled. Wave .guide V`'bent into.circular form with -aconductive radial partition 5K1l across theinterior. YIf preferred, fthe length of 'wave guide vf'rriafy rstbefclosedfat bothlends land then bentinto the formlof a circle'withthezclos'e'd ends contact, .this being the .equivalent 'for presentpurposes V-oila foi-remar guide a `radial partil-Jion.. The xrwave guideis assumed `to be capable of accommodating oscillations comprising .astanding Wave, l:the vwave form `of which zis represented the dottedlcurve 51.. VEqually v'spaced holes `.52 similar to the 'holes "41 in the:system of i2 :are provided at the antinodal v:points fof thestanding.'Wav'e configuration, there 'being of necessity a nodeatlthepa'rtition. The waveguide of-"lligg'fi .may beused 1in place .of theblock 13'. intlfre-fsy`s`tem-of 2, for example. A -In the operation-.ofl'a system employing a wave guide as illustrated ii-r1 Fig. 4, thestanding -wave may be ysustained i-n the Wave guide by means *ofl 'arotating electron beam entering, the guide periodically through thesuccessive holes 52. lThe Writing velocity Yof the v'beam Jm'ust Ibeequal 'to the velocity of propagation of the wave motion 'causing thestanding Wave 5I. Or, rconsideri-ng the standing iwave to -be composedof `two traveli-'ng Waves traversing the guide in `opposite direc'tionswith equal Velocity, the Writing velocity ofthe beam must lequalthephase velocityef the traveling Waves. v I A numerical example at thispoint will aidin the explanation as Well 'as indicate .how asystemfbased'onlig. 4 maybe designed fori-given input and youtputfrequencies. 'Suppose vthat V-a Awave guide `with a' particular shapeand .size iof cross sectionhas lbeen selected, -for example, the

quer-mies will be assumed, as before, to ben'500 megacycles and v110,000megacycles per second, respectively. "The frequency-ratio being thusdetermined, 'the circumference Aof the circle upon which'theValletues752 lie is accordingly .fixed at Dwave-lengths,'measured in theguide. To nd the actual'length'o'f the circumference, a knowl- 'edge'o'f 'the wave-.length vin the guide .is required, 'or of 'the zpha'sevelocity in the guide., from which the Wave-.lengthisreadilyvcalculated. It is known from. the theoryiof` guided wavetransmission that thefphase fvelocity .in a hollow guide with con'-ductive walls isal'wa'ys .greater than thefvelocity of lightfor allfinite frequencies which the guide will freely transmitand that thephase velocity approaches the velocity of;light asymptotically as thefrequency isincreased. The phase velocity in a particular Wave guidewill depend upon the shape and sizeof cross section' as well as upon thedesired output frequency. 'The value of the phase velocity may beobtained most readily in many cases from measurements, by known methods,,oiithe Wave-length of standing waves in a length of the actualguide.The sample upon which the measurements are madeV .may be straight andthe results willrapply with sufclent approximation to the same guidebent in the form of a circle. If formulae are available for the type ofguide employed it is also possible to calculate the wave-length andphase Velocity. For the purposes of the present example it will beassumed that the phase velocity in the guide is known to be 1.25 timesthe velocity of light. Twenty wave-lengths of a wave propagated .at 1.25times the velocity of light evidently makel a length equal to four-fthsof 20, or 16 wavelengthsof a wave propagated with the velocity oflight.V'Ihe circumference of the wave guide is, accordingly, Llicentimetersandthe diameter is approximately 15.3 centimeters, or 6 inches. The numberof apertures provided will be 40, that is, one foreach antinodal voltagepoint. In practice, the wave guide may be brought into precise resonanceat a harmonic of the input frequency by tuning, as for example, byadjusting the volume of the resonant cavity by `any suitable knownmeans.

In order to avoid loss of efficiency arising from the fact that theelectrons during so large a proportion of the time strike the outsidesurface of the wave `guide between the holes, a continuous slotextending around the entire means circumference maybe employedinstead oftheholes. Such a slot is illustrated in the wave guide shown in Fig. 5.Since the electron` beam moves along this slot with a writing velocityequal to the phase velocity in the guide, the beam will continuouslyenterthe guide against an opposing electric eld. That this is so may bevisualized by considering againthe equivalence of the standing wave anda pair of traveling waves going in oppOsite directions. The beam keepsin -a constant phase relationshipwith the traveling wave going in thesame direction, thus continually transferring energy `Vto that wave. Atthe partition, the traveling wave is reflected and merges with the wavetraveling in the opposite direction, thereby transferring some of itsenergy to the other traveling wave.-

-A wave guide of the cross section illustrated in Fig.o5, approximatelycomprising two circular sectors, is adapted to permit the electrons topass through the guide in a timecomparable with the periodic timeof theoutput frequency. For example, a-,gap of about 1/20 inch may beadvantageouslyemployed with an output frequency of 10,000 megacycles. Y

. In the arrangement of Fig. 4 it is feasible to omit the partition 56and allow the rotating beam to enterthe guide through a continuous slot.Without the partition, the guidefcan sustain a traveling wave whichprogresses continuously around the circumference. The `traveling wavemay-be maintained by continuous abstraction of energy from theelectrons, which come into the guide at a point of maximum opposingelectric field. Thus the transfer of energy from vthe beam totheelectromagnetic wave in the guide is substantlally continuous. This.method of sustaining a, traveling wave in an endlesswave guide bymeansof a rotating electron beam is dis-A closed in a copending applicationof R. V. L. Hartley, Serial No. 385,629, led March 28, 1941,' andassigned to thev assignee of the present application.` j Y Fig. 6.showsan embodiment of the general arrangement described in connection withFig. l with certain modifications, the system in some respectsresembling that illustrated inFig. 2. vThe wave guide rvshown inFig. 6has substantially the same cross section as the resonator in blockv I3Aof Fig. 2. Instead of holes for the electron beam to entera continuousslot'is shown, the central portion of the block being supported yin anysuitable manner as, for examplaby radial rodsv which cross the path ofthe electron beam but intercept relatively few electrons. Advantage maybe taken of an arrangement disclosed in the above-cited Hartleyapplication for increasing the circumference swept out by theelectronbeam while using relatively low energizing potentials. Electrodes 60 and6I, respectively, provide between thema conical slot through which theelectron beam passes.V A steady electric field impressed between theelectrodes 66 and 6I by a battery 62 or other suitable source ofelectromotive force, causes an outward bending of the electron beam,thereby increasing the divergence of the beam from vthe axis. Reversalof the polarity ofthe source 62 would, of course, result ina decreaseinthe diameter of the circle swept out by the electron beam. A pair ofelectrodes 63 and ,654 separated by a circular slot may be placednearthe resonator and polarized somewhat positive with respect to theconductive surface of the wave guide by a battery 65 or other suitablesource of electromotive force, in order that any secondary electronswhich may be emitted due to the electron beam striking any portion ofthesurface of the wave guide may be attracted to and collected by theelectrodes 63 and 64. The circular slots in the electrode systems 60, 6Iand 63, 64 are arranged to register with the slot in the wave guide sothat the electron beam may readily pass through al1 three slots.

One or both ofthe electrodes 63 and 64 may also be employed to eifectanautomatic control ofthe deflection of the electron beam. For example, aresistor 66 may be inserted in series with the source 65 and thepotential drop` across the resistor 86 may bearranged to subtract from`the potential difference between the electrodes 60 and 6|, thepotential across the resistor partially offsetting theelectromotiveforce of the source 62. When the input or deflection amplitude changes,tending to throw the beam out of register'with the slot in the waveguide `and thereby tending to reduce .the current through-the cavity,the current intercepted by one or the other of the electrodes S3, 64 Aischanged, for example, in this Vcase the current to electrode 64.supposing that lthe current to the electrode Ellis increased, then thepotential difference across the resistor 65 will also be increased and,accordingly, there will be a decrease in the potential differencebetween the electrodes 60 and 6l. The latter will tend to draw the beamcloser to the axis and away from the electrode S4, thereby decreasingthe current to that electrode and tending to minimize the resultantchange in the deectionvof the beam.; `The control potential of theresistor 66 may beemployed, if desired, to control the amplitude'of` theyhigh frequency, input to the 9 plates l1, I8, I1' and I8 with similareffect upon the deflection of the beam. Likewise, it is feasible toemploy electron current intercepted by the electrodes 6l] and 6I tosecure correcting potentials which may then be applied to the electrodesI1, I8, I1 and I8' as before.

What is claimed is:

1. A harmonic generating system comprising a source of waves of givenfrequency, means to provide a beam of moving charged particles, aresonating chamber resonant to a harmonic of said given frequency andhavingA an aperture in its wall permitting access of said beam into theinterior of the chamber, means energized by said source of waves ofgiven frequency to direct said beam into said aperture to react with anelectromagnetic wave within said resonating chamber, and means tocontrol said beam directing means to limit the phase of said reactionsubstantially tion and cause said beam to sweep once per cycle of thegiven frequency over said aperture and through an arc relatively greatcompared with vand including the arc subtended by said aperture at thesaid point of deection.

3. A harmonic generating system comprising a source of waves of a givenfrequency, a resonating chamber tuned to a harmonic of said givenfrequency and having an aperture in its wall, means to vprovide a beamof moving charged particles, and means energized by said source of wavesto deflect said beam to and fro across said aperture and substantiallyabout a, xed point of deflection once per cycle of the given frequencythrough an arc relatively greater compared with and including the arcsubtended by said aperture at said point of deflection.

4. A harmonic generating system comprising a source of waves of a givenfrequency, means to provide an electron beam, a hollow resonator coaxialwith said electron beam and having conductive walls and anaxial'aperture, said resonator being tuned to a harmonic of said givenfrequency, and means energized by said source of waves of givenfrequency to deflect said beam to and fro across said aperture andsubstantially about a fixed point in the axis once per cycle of thegiven frequency through an arc relatively large compared with andincluding the arc subtended by said aperture at the said point ofdeflection.

5. An electron beam system comprising a cathode, means to formr anelectron beam from electrons emitted by said cathode, a targetcontaining an aperture, means to deflect said beam about a substantiallyfixed point of deflection through an arc relatively greater than and in-5 cluding the arc subtended by said aperture at said point ofdeflection, means to impress a relatively large potential differencebetween said cathode and said target to accelerate electrons toward saidtarget, means to shield said electron beam from said target throughoutthe major portion of the arc of deflection, said shielding means havingan axial opening permitting access of the beam to the aperture in thetarget, and means to impress a relatively small potential differencebetween said cathode and said shielding means to collect electrons withlow energy dissipation on said shielding means.

6. A harmonic generating system comprising a source of waves of a givenfrequency, a toroidalshaped resonating chamber tuned to the harmonic ofsaid given frequency, said resonating chamber having a plurality ofapertures uniformly spaced about its periphery, means to provide a beamof moving charged particles, and means energized by said source of wavesof given frequency to sweep said beam over said apertures in rotationonce per cycle of said given frequency, the ratio of the number of theharmonic to the number of apertures having an integral value. '7. Aharmonic generating system comprising a source of waves of a givenfrequency, a hollow resonator with conductive walls, the cavity of whichresonator is a gureof revolution about an axis outside the cavity, saidresonator having a resonant frequency that is a multiple Aof the givenfrequency, and said resonator having a plurality of aperturescommunicating with the cavity of the resonator and uniformly spacedabout a circle concentric with said resonator, a source of a beam ofelectrons, and means energized by said source of waves of givenfrequency for sweeping said beam f electrons over said apertures insuccession at a uniform rate correlated with the -resonant frequency ofsaid resonator to sustain ll an electromagnetic wave within the cavityof said resonator at said multiple frequency.

8. A harmonic generating system comprising a source of waves of a givenfrequency, a hollow conductive wave guide closed at both ends of a 50length to support a plurality of cycles of a standing electromagneticwave of a frequency which is a multiple of said given frequency, saidwave guide being bent into circular form and provided with a pluralityof apertures lying upon a circle 55 and coinciding substantially withthe antinodal points of said standing wave, means to provide a beam ofelectrons and means synchronized with said source of waves of givenfrequency to sweep said electron beamfover said apertures in suc- 60cession with a writing Velocity equal to the phase velocity of theelectromagnetic wave in said wave guide.

JAMES W. MCRAE.

